High strength precipitation hardening heat resisting alloys



Jan. 7, 1969 MASAO KAWAHATA ETAL 3,420,650

HIGH STRENGTH PRECIPITATION HARDENING HEAT RESISTING ALLOYS Filed Sept. 17, 1964 F1q 1 l D on E 4 G C I 8 H L a 20 F A Rupture Time (in (5 u)0- NA 0.88%

R cuPe Time Hm) 3,420,660 HIGH STRENGTH PREQIPITATION HARDENING HEAT RESISTING ALLOYS Masao Kawahata, Yokohama, Kozou Yokota, Tokyo, Yukishige Fukase, Kawasaki, and Shoichi Katoh, Tokyo, Japan, assignors to Nippon Yakin Kogyo Company Limited, Tokyo, Japan, a corporation of Japan Filed Sept. 17, 1964, Ser. No. 397,103

Claims priority, application Japan, Sept. 20, 1963,

1 Claim ABSTRACT OF THE DISCLOSURE A heat resisting alloy which has a high creep rupture strength, excellent toughness at temperatures up to 800 C. and excellent resistance to oxidation is prepared by adding precipitation hardening elements comprising molybdenum, tungsten, niobium alone or added with uranium and vanadium, titanium, aluminum and at least one of boron, zirconium and/ or tantalum to a base alloy having a crystal structure of face-centered cubic containing iron, chromium, nickel and cobalt.

The present invention relates to an economic super heat resisting alloy which has a high creep rupture strength used under the condition of high temperature and high stresses, such as mechanical construction members in jet engines and gas turbines and the like.

The principal object of the invention is to provide a heat resisting alloy which has a high creep rupture strength and excellent toughness at high temperatures up to 800 C. and is excellent in oxidation resistance, said alloy being produced by adding one or more precipitation hardening elements comprising molybdenum, tungsten, niobium alone or added with uranium and vanadium, titanium, aluminum, and at least one of boron, zirconium and/or tantalum, to a base alloy having the crystal structure of face-centered cubic type containing iron, chromium, nickel and cobalt.

The invention is characterized by a high strength precipitation hardening heat resisting alloy which contains as base alloy elements 0.010.5% of carbon, 570% of iron, 12-25% of chromium, 13-40% of nickel, 530% of cobalt, -1.5% of silicon and 02% of manganese; as precipitation hardening elements 0.1% of molybdenum, 0.1-5% of tungsten, 0.l5% of vanadium, 0.15% of titanium, 0.15% of aluminum, 0-5% of tantalum, 0.0010.4% of boron and/or zirconium, 0.15% of niobium and 02%, preferably 0.01-2% of uranium; the remainder iron and less than 2% of impurities in total including such elements as sulfur, phosphorus or copper, which may incidentally enter during melting process and is allowable to be contained.

In order to obtain the alloy of the invention successfully, it is preferable to remove gaseous content of the nited States Patent alloy by melting in vacuo and manufacture a clarified steel containing less than 0.005% of oxygen and less than 0.01% of nitrogen, and the more excellent high temperature strength characteristics can be obtained in accordance with the invention by the solid-solution treatment of the alloy at 1,000-l,200 C. for 110 hours and thereafter by one or several age-hardening treatments at 600-980 C. for a suitable period of time within 1-50 hours.

As typical alloys used hitherto for such purpose of applications and having an excellent high temperature strength characteristics, there have been known A-286 and W545 as iron base alloy, and alloys known by trademarks of Nimonic A, 90, and 100, M-2S2, Inconel X and Waspalloy as nickel base heat resisting alloys.

Among above mentioned alloys, heretofore known iron ibase heat resisting alloys, i.e. A-286 and W545, of which basic compositions are nickel-chromium-iron alloys developed from stainless steel of austenite series, have disadvantage that they decrease in strength to a great extent at high temperatures above 700 C.

The alloy according to the invention is characterized in that cobalt is added to a nickel-chromium-iron base alloy, and niobium and optionally uranium is coexisted in a nickel-chromium-cobalt-iron base alloy of which base alloy composition is modified and other various hardening elements are added to improve the high temperature strength characteristics and to advance the practical temperature limit by ca. 50 C.

The addition of cobalt to nickel-chromium-iron base alloy results in the effective improvement of the strength characteristic because of the strengthening of the matrix, the adjusting of precipitated phase by the addition of various hardening elements and the changing of the distribution of precipitated phases.

For the additional purpose of strengthening the matrix, titanium and aluminum as precipitation hardening elements are added and, in addition, various elements such as molybdenum, tungsten, vanadium, niobium and optionally uranium are added. The principal causes to improve further the high temperature strength characteristics after a long period of time are mentioned: that the pre cipitation of brittle phase consisting of nickel-titaniumaluminum or iron-chromium-molybdenum as principal component, which impairs greatly the high temperature strength of nickel-chromium-cobalt-iron base alloy, is prevented by coexisting, in particular, niobium and optionally uranium, and that the precipitation distribution condition of the precipitation phase of nickel-titaniumaluminum, which is the principal cause for strengthening, is improved by the precipitation phase consisting of ironniobium-uranium as principal component.

Accordingly, a new alloy which is superior to iron base alloys known by the trademarks of A-286 and W-545, nickel base alloys known by the trademarks of Nimonic 80A, 90 and Inconel X has the strength characteristic satisfactorily comparable to the high temperature strength of nickel base alloys, Nimonic (trademark) 90, and M-252, has been found according to this invention.

TABLE 1.CIIEMICAL COMPOSITION Name of alloys C Si Mn Cr Ni Co Mo W V Nb-l-Ta U Ti Al B Fe Known alloy A (Ar-286) 0. 08 0.4- 1. 13.5 24.0- 1.00- 0.10- 1.75- 0. 35 Bal Known alloyB (LON-155)-.. 0.20 0.6 1. 00- 20.0 18.0- 18.0- 2.50 2.0- 0.75- Bal.

Known alloy C (Ineonel X). 0.04 0.3 0.7 15 73 0.9 2. 0 6 7 Known alloy D (M252) 0.15 0.5 0 5 19 55 10 2.5 1 0 2.0

Alloy of the invention E 0.08 0.65 1.49 16.00 20.88 5.20 2.28 1 10 0.51 0.82 2.10 0.69 0.12 Bal. I612 2 Alloy of theinvontion F 0.11 0.61 1.48 17.21 20.19 9.91 2.14 1.38 0.60 0.89 1.58 0.49 0.10 Bal. I612 2 I Alloy oi the invention G 0.11 0.28 Tr. 16.01 20.15 20.58 2. 35 0.81 0.59 0.83 2.80 0.70 0.12 Bal. 1612 2 Alloy of the inventionH 0.11 9.25 Tr. 16.00 20. 20.25 3.01 0.98 3.10 2.17 0.11 Bel. g1 2 Alloy of the invention I 0.06 0.32 1 02 16.05 20. 48 20.98 2.81 0.91 0.62 0.85 0 80 3.01 0.80 0.05 Bal. 1612 2 a Examples of alloys according to the invention are illustrated in detail hereinafter in comparison with the high temperature strength characteristics of a conventional iron base and nickel base alloys available on the market. The compositions and test results of the conventional alloys A, B, C and D mentioned for comparison are described with reference to the values of ASTM, STP No. 160 (1954).

As shown in Table 1, in the alloy according to the invention, hardening elements such as molybdenum, tungsten, vanadium, niobium, uranium, titanium, aluminum and boron etc. are added to nickel-chromium-cobalt-iron base alloy, in which 530% of cobalt is added to nickelchromium-iron base alloy to reduce the high content of expensive nickel, whereby the high temperature strength of the alloy is further improved by the combined effect of the strengthening of solid solution of base alloy, the precipitation hardening by carbide and the precipitation hardening by intermetallic compound.

Among a number of heretofore practically used heat resisting alloys, the air-melted alloy increases in its high temperature strength by adding many hardening elements as described above, but decreases considerably its high temperature toughness and at the same time has a deadly disadvantage of making forging diflicult.

As shown in Table 1, the alloys E, F, G and I according to the invention are greatly improved in forgeability and toughness, because a clean steel of lower oxygen and nitrogen content can be produced by utilizing the feature of induction vacuum melting method to perform a suflicient deoxidation treatment.

According to the invention, a mixture of aforementioned hardening elements is added to the base alloy in such an amount that the resultant alloy may have an excellent high temperature strength while maintaining the forgeability and toughness thereof at sufficiently high levels.

For improving the high temperature rupture strength, the heat resisting alloy according to the invention can be improved in its high temperature toughness and creep strength by subjecting to a sufficient solid solution treatment as pretreatment and thereafter by applying a precipitation hardening treatment. When the alloy according to the invention is subjected to a sufficient solid solution treatment at a temperature within the range of 1,000- l,200 C. and to a suitable age-treatment at a temperature within the range of 600980 C., the better characteristics can be obtained. The sufficient solid solution treatment can not be performed below 1,000 C., and over 1,200 C. the eutectic structure is formed to impair the high temperature strength.

At a temperature below 600 C. the age-treatment results in practically no effect of precipitation hardening treatment and at a temperature above 980 C. there occurs resolution of the precipitated phase so that the agetreatment gives no effect.

The test results of the alloy according to the invention have been obtained by testing a sample which is subjected to the solid solution treatment at 1,l50 C. for 2 hours and thereafter to the optimum age-treatment at 760 C. for 20 hours.

For a better understanding of the invention reference is taken to the accompanying drawing, in which FIG. 1 shows the characteristic diagram of the creep rupture strength at 732 C. and

FIG. 2 shows the characteristic diagram illustrating the effect of uranium and/or niobium added to the alloy.

Referring to FIGS. 1 and 2, the curves E, F, G, H and I represent the mechanical strengths of novel alloys denoted by the same reference characters in Table 1 and 2, which have equivalent or better mechanical strengths compared with those of conventional nickel alloys, despite the fact that content of nickel is greatly reduced in alloys of the invention. Especially, the alloy I containing both niobium and uranium has excellent mechanical strength exceeding that of any conventional alloy.

The reason for such excellent mechanical strength of the alloy of the invention is due to the fact that the precipitation phase of intermetallic compound of which main component is niobium or niobium plus uranium makes better the distribution of precipitation phase of nickel, aluminum and titanium by the addition of niobium or niobium and uranium.

The addition of both niobium and uranium results in the improvement of rupture strength for long time of alloy according to the invention depending on the relatively fine distribution of precipitation phase consisting of principally such elements and the decrease of coagulate tendency.

Table 2 shows the comparison of creep rupture strength at 732 C. and 815 C. for and 1,000 hours, and that the alloy according to the invention is superior to any previous alloy and reference examples.

Even in the alloy according to the invention, the rupture strength at 815 C. is substantially comparable with that of the known alloy C and it seems that strength of the alloy according to the invention at a temperature above 800 C. decreases more than that of nickel base alloy.

732 C. (kg/mm?) 815 C. (kg/mm?) Name of alloy 100 1,000 Name of alloy 100 1,000 hours hours hours hours Known alloy A... 25.0 14. 6 Known alloy A. 8. 8 5. 6

Known alloy B... 22. 17. 0 Known alloy B- 13. 0 9.0

Known alloy O 35. 0 28. 0 Known alloy 0 19.6 12. 6

Known alloy D 41.0 30.8 Known alloy D 23. 8 12.6

Alloy of the in- Alloy of the invention E 29. 6 20.0 vention E 9.0

Alloy oi the in- Alloy of the Invention F 28. 7 22.0 vention F 8.0

Alloy of the ln- Alloy of the invention G 40.0 30.0 vention G 15.0 8.5

Reference Exam- Reference Example H 36. 0 22.0 ple H 12.0 7.6

Alloy of the in- Alloy of the invention I 43. 0 32. 0 vention I 17. O 9.

The reason of limitation of the range of compositions of the alloy according to the invention will be explained in the following:

(1) Carbon 0.01-0.5

The alloy according to the invention necessitates the carbon content 0.0l0.5% since it is difiicult to melt at the carbon content below 0.01% and to get the improved creep strength, whilst at above 0.5%, the high temperature toughness becomes worse and it is difficult to produce.

2 Chromium 12-2s% Chromium is an element that is added for the purpose of giving the oxidation resistance to the alloy of the invention, and at below 12% it does not give a sufficient oxidation resistance, whilst at above 25% of Cr expensive nickel must be added in greater amount to make an alloy of face-centered cubic structure.

(3) Nickel 13-40% Nickel is an important element necessary to make the alloy according to the invention a face-centered cubic structure and the lower limit should be 13% to give a suflicient face-centered cubic structure after adding a number of precipitation hardening elements. Though large amounts of nickel are preferred, the upper limit of 40% is defined as a necessary limit since it becomes more expensive.

(4) Cobalt 530% Cobalt is an element that serves to make the matrix of alloy according to the invention a face-centered cu'bic structure and is also necessary for strengthening the matrix and adjusting the precipitation. It is also a principal element that serves to improve the strength characteristics and practical temperature limit in strength of the alloy according to the invention. Its effect is little at below 5% and the upper limit should be 30% to maintain the alloy according to the invention as cheap as possible.

(5) Silicon 0-l.5

Though silicon is a necessary deoxidation component in the production of heat resisting steels, the upper limit is taken as 1.5% since it impairs the cleanness and forgeability of steel at over 1.5 The less silicon, the better the high temperature toughness. The lower limit is taken as 0, since it can be regulated up to 0% if vacuum melting processes are employed.

(6) Manganese 02% Manganese is also a necessary deoxidation component in the production of heat resisting steels and it is allowable up to the upper limit of 2% which may be introduced as impurity. The lower limit is taken as 0 according to the same reason as in silicon.

(7) Molybdenum, tungsten and vanadium 0.l5%

Molydenum, tungsten and vanadium are elements to be added for strengthening the solid solution of matrix and forming carbides, and it is not preferred to add more than 5% because the matrix of face-centered cubic structure becomes unstable, the harmful precipitation phase will be formed and the oxidation resistance is impaired.

(8) Niobium 0.15%

Niobium is likewise a carbide-forming element and more especially an element that forms an intermetallic compound by combining with iron and improves the precipitation distribution. However, at more than 5%, the forging property is injured and makes difficult the production of alloy and rather the high temperature strength may be lowered, so that the upper limit is settled as 5%. The effect by adding niobium is not produced at lower than 0.1%.

(9) Uranium 0.01-2.0%

Uranium is an element that gives the same effect as niobium. The addition of more than 2% impairs the forging property and the effect of uranium is not shown at below 0.01%.

(10) Tantalum 05% While tantalum is an element that shows the same eflect as niobium, it impairs the high temperature toughness and forging property and makes difficult the production of alloy when it is contained more than 5%. The lower limit is made as 0 because it may not be contained, while it is optionally introduced in a small amount accompanied with the addition of niobium.

(11) Titanium and aluminum 0.15%

Titanium and aluminum are principal elements for forming an intermetallic compound of nickel, aluminum and titanium which are the most effective and important precipitation phase elements in the alloy according to the invention. However, when more than 5% of any one of these two elements is added the production of alloy is made difficult, and the desired effect can not be obtained at lower than 0.1%.

(12) Boron and zirconium 0.0010.4%

While boron and zirconium are the necessary elements that give the high temperature toughness to the alloy according to the invention, it impairs the forging property and degrades the oxidation resistance when it is contained more than 0.4%. Both boron and zirconium may be used simultaneously and any one of them causes the effect. From the reason described above, 0.00l-0.4% of boron and/ or zirconium are selected as addition elements.

As shown in the above example, the alloys according to the invention have sufficiently high mechanical strength and oxidation resistance properties for mechanical constructions used under high stresses at a high temperature between 600 C. and 800 C., enduring the loaded condition for a long period of time, possesses a very excellent performance and is superior to the commercial iron base alloy A-286 and W545. The alloy is improved by about more than 50 C. in the practical temperature limit of strength and can "be produced at lower cost so that it gives greater advantage in industry compared with a nickel base alloy.

What we claim is:

1. A high strength precipitation hardening heat resisting alloy, consisting essentially of base alloy elements 0.01-0.5% of carbon, 12-25% of chromium, 1340% of nickel, 530% of cobalt, up to 1.5 of silicon and up to 2% of manganese; as precipitation hardening elements 0.1-5% of molybdenum, 0.l5% of tungsten, 0.1-5 of and zirconium, (Ll-5% of niobium, and .01-2% of uranium to improve stress rupture properties; and balance essentially iron.

References Cited UNITED STATES PATENTS Franks 75128 Loveless 75124 Eisermann 75128 Wagner 75128 8 Harris 75128 Pitler 75124 Heydt 75171 Lillys 75124 L. DEWAYNE RUTLEDGE, Primary Examiner.

PAUL WEINSTEIN, Assistant Examiner.

US. Cl. X.R. 

